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<br />001G71 <br /> <br />the basin was significantly modified and flow restrictions were imposed on releases from the two <br />72-inch hollow-jet valves. The conclusions of the study, presented in Hydraulics Laboratory <br />Report HYD-573 [13], are: <br /> <br />1. The model confirmed that damage to the prototype stiI/ing basin was caused by <br />circulation of abrasive material which moved into the basin from the downstream <br />channel. The severe damage in the upstream portion of the basin probably occurred at a <br />discharge of approximately 1,840 cfs (cubic feet per second) with reservoir elevation <br />5965 (about 245 feet of head on the valves). Both valves were about 30 percent open. <br />Damage to the downstream portion of the horizontal basin floor probably occurred with <br />both valves 100 percent open at approximate reservoir elevation 6000. Discharge was <br />about 3,940 cfs. <br /> <br />2. The original design of the stiI/ing basin, with converging wedges and center dividing <br />wall, could not be improved upon with respect to efficiency in energy dissipation and ! <br />stability of turbulent action in the basin. However, the high efficiency of the basin '! <br />resulted in areas of intense turbulence which made the basin more susceptible to damage / <br />by the circulation of abrasive materials. The studies also indicated that at a discharge of <br />3,940 cfs, the frequency of pressure fluctuations matched the natural frequency of the <br />prototype wall. Resonance of the wall would result in moments at the base of the wall <br />several times greater than those used in the design. <br /> <br />3. The model showed that the prototype outlet works could be operated at a discharge of <br />1,000 cfs for reservoir elevations below approximately 6000 without causing additional <br />damage to the stiI/ing basin. Periodic inspections by divers during operation under these <br />conditions supported this finding. <br /> <br />4. Several modifications to the original design were tested to develop a stiI/ing basin <br />which would allow sustained operation of the outlet works without additional damage to <br />the basin. The converging wedges were removed to eliminate confined areas of intense <br />turbulence. A 2-1/2:1 chute was installed at the upstream end of the basin to induce <br />lateral spreading of the jets in order to maintain a high bottom velocity and sweep <br />material from the basin floor. The center wall was removed with the stipulation that the <br />outlet works should not be operated with only one valve open. A 12-inch layer of <br />concrete was added to the inner surfaces of the outside walls and basinfloor to repair the <br />damaged areas and provide additional strength. <br /> <br />5. The model showed that abrasion damage could occur in the modified basin, even <br />though this tendency was reduced Also, the possibility remained that channel material <br />might enter the basin during low discharges. Therefore, the downstream channel was <br />paved for a distance of about 140 feet and a rock trap was provided between the paved <br />area and the stiI/ing basin. <br /> <br />14 <br />